1. Field of the Invention
The present invention relates in general to zinc-bromine battery systems, and, more particularly, to a device and method for the re-acidification of an electrolyte stream in a zinc-bromine flowing electrolyte battery.
2. Background Art
The original concept of utilizing the properties of zinc and bromine in a battery system was patented over 100 years ago in U.S. Pat. No. 312,802. Generally, the battery system has a negative flow loop and a positive flow loop, as well as a separator of some kind in-between. The zinc-bromine electrolyte is circulated through both loops, depositing zinc at the negative electrode, and creating aqueous bromine at the positive electrode, all while creating a voltage difference between the two electrodes. The zinc is collected as a solid, while the aqueous bromine forms a second liquid phase and is separated from the flowing electrolyte.
Utilizing a circulating electrolyte system, Zinc-Bromine batteries have significant advantages, including ease of thermal management and uniformity of reactant due to electrolyte flow, operation of the system at ambient temperature, rapid system charging, complete system discharging, good specific energy of reactants, and a system that is generally constructed from low-cost and readily available materials. The system did not gain immediate commercial acceptance, however, due to the formation of zinc dendrites upon deposition of zinc at the negative electrode, impeding the flow of electrolyte, and due to the solubility of bromine in the zinc-bromine electrolyte, causing a cell short circuit.
In the 1970s, Exxon Corp. and Gould Inc. developed techniques that attempted to inhibit the formation of zinc dendrites upon deposition at the negative electrode. Upon operation, the cell could now be operated for significantly longer periods of time without the previous inhibited flow. The zinc-bromine battery was now a commercially reasonable means of storing and recovering power. However, current operation of zinc-bromine batteries still contain significant problems.
Current operation of a zinc-bromine cell requires specific parameters for continuous operation. Among these requirements is one that the system be operated at or near a pH of two. This requirement exists because at higher pH levels mossy zinc plating develops, as well as bromates within the electrolyte solution. Alternatively, at lower pH values, zinc corrodes at an increasing rate. Although the system reactions do not themselves affect pH, overcharging of the cell during cyclical operation may electrolyze water, creating gaseous hydrogen and hydroxide ions in the water, raising the pH.
Therefore, it is an object of this invention to create a device and method for the re-acidification of the zinc-bromine electrolyte stream in a flowing electrolyte system to, in turn, facilitate longer and more efficient continuous operation of the battery.
It is a further object of this invention to create a means for re-acidification utilizing the products of the current battery system so that an ongoing and steady-state system may be developed.